The present invention relates to drill bits having inserts pressed into one or more cutter members or cones and, more specifically, it concerns a rock bit having hardfaced wear protection on the cutter members.
During the drilling of boreholes, rock bit cutter members constantly operate in a highly abrasive environment. This abrasive condition exists during drilling operations wherein either a drilling mud, air, or gas is utilized as the medium for cooling, circulating, flushing, and carrying the cuttings from the borehole.
Drill bit life and efficiency are of prime importance in the drilling of oil and gas wells, blast holes, raise holes or other types of boreholes since the penetration rate is related to the condition of the bit. When harder formations are encountered during drilling of the borehole, a bit having carbide inserts projecting from the body of each of the cutter members is generally utilized because of the ability of the inserts to penetrate the hard formations. However, the carbide inserts are mounted in a relatively soft metal that forms the body of each of the cutter members. The relatively soft metal cutter body which holds the inserts in place is abraded or eroded away relatively easily when subjected to the highly abrasive drilling environment. This abrasion or erosion is primarily due to the presence of relatively fine cuttings from the formation that have not been carried out of the borehole, the direct blasting effect of the fluid utilized in the drilling process, and the rolling or sliding contact of the cutter body or cone shell with the formation.
The wearing away of the cutter member body is usually most pronounced on the inner and outer edges of the lands of the cutter surface immediately adjacent the insert and the groove between one row of inserts and another on the cutter member. For every two consecutive rows of inserts on a cutter member, one is considered the outer row, the other the inner row. The heaviest wear on the cutter member surface lands is usually most pronounced on the inner edges of the outer rows, and on the outer edges of the inner rows. Consequently, the innermost row on the cutter member will predominantly wear on the outer edge of the land, the gage row predominantly on the inner edge of its land, and the rows in between the two wear on both the inner and outer edges of their lands.
When the material supporting the inserts is eroded or abraded away to a sufficient extent, the drilling forces being exerted on the inserts when they engage the formation either break the inserts or force them out of the cutter member altogether with the result that the bit is no longer effective in cutting the formation.
When drilling many of the softer abrasive formations where the bit is able to penetrate at an extremely high rate, it can be expected that the individual cutting inserts can penetrate entirely into the abrasive formation causing the formation to come into contact with the cutter body or cone shell. When this cone shell contact occurs, the relatively soft cone shell material will erode away, namely at the edges of the surface lands, until the buried portion of the insert itself becomes exposed and the retention ability in the cone shell is reduced ultimately resulting in the loss of the insert and reduction of bit life.
The inserts are retained in the cutter member by the "hoop" tension generated when the insert is pressed into a drilled hole in the cutter member body. Accordingly, any method utilized in attempting to alleviate the erosion of the cutter member must take into consideration that the "hoop" tension holding the insert must be retained.
U.S. Pat. Nos. 3,461,983 and 3,513,728 issued to Lester S. Hudson and Eugene G. Ott disclose a drill bit having a plurality of cutter members with hard inserts in holes surrounded by hardfacing. These patents describe a method of manufacturing the drill bit wherein holes are drilled in the cutter members and, then, the holes are plugged and the hardfacing material is applied to the surface around the plug. After the hardfacing material has been permanently bonded to the surface, the plugs are removed and the hard inserts are pressed into the holes to complete the apparatus.
It has been found that the method described in the above-mentioned patents is impractical because of the tedium and the economics of placing plugs into the numerus holes on a cutter member. In addition, it was also found that the heating process which bonds the hardfacing to the member surface caused the pre-formed holes to warp or otherwise become out of round. This out of roundness lead to an inconsistency in the "hoop" tension that holds the inserts in the cutter member and ultimately to the loss of inserts.
It also has been found impractical to press the inserts into the cutter before applying the hardfacing since the utilization of heat to adhere the hardfacing material to the surface of the cutter member relieves the "hoop" tension in the cutter member.
Previous attempts to hardface cutter members before drilling the holes lead to difficulty since the hardfacing was placed on the cutter member surface where the inserts were to be located. Penetrating this hardfacing material proved to be difficult and impractical. Even when holes were successfully drilled through the hardfacing material, pressing the inserts into the holes resulted in cracks in the cutter member immediately surrounding the inserts. The cracks relieved the "hoop" tension so that the inserts were not retained adequately.
Certain cutter shell areas were not expected to experience wear because it was thought that during normal drill bit operation the cutter shell would not come into contact with the abrasive formation. However, with the increased use of tooth-shaped insert bits for use in softer formations where the full insert extension penetrates entirely into the formation, this expectation is not applicable. Current high penetration rates with these types of insert bits have made cutter shell erosion an increasingly significant factor in limiting bit life.
In light of the foregoing, an economical, uncomplicated, and aesthetically pleasing method of protecting the vulnerable cutter shell is sorely needed.